Earth Planet. Sci. Lett. http://doi.org/q2h (2014)

Geographical variations in the coupling between the tectonic plates and the underlying mantle are poorly constrained. Seismic data from the American Midwest now show that mantle flow is strongly coupled to the plate beneath Minnesota and Iowa, where the continent has a deep root.

Ken Dueker at the University of Wyoming, USA, and colleagues seismically imaged the structure of the rigid plate in the midwestern United States along with the underlying, flowing asthenospheric mantle. The North American Plate is thicker in the east, and has a deep root. The images show that where the plate thickens, passing seismic waves travel faster in one direction than perpendicular to it. The researchers explain this directional preference as a result of the acceleration of the flowing mantle as it is forced to move beneath the thick continental root, in the same way that air accelerates as it flows around the curved wing of an aeroplane. The acceleration of flow induces strain in the mantle rocks, causing minerals to align with the direction of flow and generating the observed fast seismic wave speeds.

Seismic investigations of mineral alignment could then help constrain the degree of coupling between the tectonic plates and underlying mantle flow elsewhere, too.